The present invention pertains to the radio communication art and, in particular, to a sectored antenna receiving system.
Several types of sectored antenna receiving systems have been developed for application in the radio frequency communication art. Such systems are commonly used in applications wherein the remote transmitter whose signal is to be received may be positioned in any one of multiple locations. Thus, such systems are normally designed in moving vehicle type applications. By using a sectored antenna array, as opposed to, for example, an omnidirectional antenna, the signal to noise performance can be significantly enhanced thus producing a superior communication system.
The prior art sectored array antenna systems, insofar as they relate to the present application, fall into two classes. The first class contemplates a multiple sector, single receiver arrangement in which the receiver is controlled by a sampling routine to continuously scan all sectors. Hence, for the condition wherein there is no received signal on any sector, the routine calls for the receiver to sample each sector for an equal period of time. Once a transmitted signal is detected on a sector, the duty cycle is changed whereby the receiver dwells on that sector for a disproportionate period of time. Nonetheless, the receiver continues to constantly sample the other sectors in anticipation of a stronger signal thereon.
The second class of sectored array receiving systems employs a pair of receivers. A search receiver constantly scans each sector in the array in an attempt to find that sector which receives the best signal. A service receiver is coupled to that sector with the best signal until the search receiver discovers that another sector is receiving a superior signal, at which time appropriate antenna switching is accomplished to the service receiver.
Such prior art scanning systems of the first class type have proven effective for applications wherein the remote transmitter is likely to jump from the coverage area of one sector to another sector in a reasonably short time as, for example, in the near range aircraft communication. However, for an application such as in land mobile systems the length of a communication message is sufficiently short compared to the coverage area of a sector and the speed of the vehicle such that a particular sector is likely to produce the best signal for the duration of the message. Such first class type prior art scanning systems are not desirable in applications such as the land mobile type since they necessarily require a constant switching from one sector to another resulting in an annoying gap in communication.
The prior art systems of the second class type have proven undesirable in that they require a pair of receivers. This significantly increases the cost of the system as well as system size while necessarily decreasing system reliability due to the duplication of parts.
In an application entitled "Sectored Antenna Receiving System," filed the same day as the original parent of this application and issued July 18, 1978, to Craig et al., U.S. Pat. No. 4,101,836, another approach is disclosed. In this system a multiple sector antenna, signal receiver arrangement is controlled by a sampling routine to continuously scan all sectors. When a transmitted signal is detected on a sector, the sampling routine activates to a second mode wherein scanning continues for a predetermined interval. At the end of the interval, scanning is inhibited and the receiver is coupled to that sector which produced the strongest level during the second mode scanning cycle. Once the signal level from the selected sector falls to a minimum level for a given time period the system reverts to the first scanning mode. This application is related to the subject matter disclosed in the above-mentioned application and is an improved sectored antenna receiving system.